Avalanche Forecast

Issued: Jan 18th, 2018 5:01PM

The alpine rating is high, the treeline rating is considerable, and the below treeline rating is considerable. Known problems include Storm Slabs and Persistent Slabs.

cgarritty, Avalanche Canada

South Rockies.

Sustained strong winds will accelerate slab formation and keep loading lee terrain even when the snow stops falling. Storm slabs should be primed for human triggering on Friday.

Summary

Confidence

Moderate -

Weather Forecast

Friday: A mix of sun and cloud with possible isolated flurries. Moderate southwest winds. Alpine high temperatures of -7.Saturday: A mix of sun and cloud. Moderate southwest winds. Alpine high temperatures of -7.Sunday: Mainly cloudy with scattered flurries and 1-4 cm of new snow. Strong southwest winds. Alpine high temperatures around -8.

Avalanche Summary

No new avalanches have been reported since Thursday's storm, but heavy snowfall has been obscuring visibility into most terrain. Looking forward, expect ongoing strong winds to continue loading lee aspects even as snowfall tapers off. These winds will also be promoting slab formation and reactivity.

Snowpack Summary

Up to 30 cm of new snow from Thursday has buried both a new layer of feathery surface hoar that has been developing in sheltered areas as well as sun crust on solar aspects. The new snow will need time to form a reliable bond to this previous surface. Beneath the new snow, recent strong winds (mainly from the southwest) scoured windward slopes and formed wind slabs that are now buried in lee terrain at higher elevations.An unstable weak layer from mid-December (predominantly feathery surface hoar crystals and/or a sun crust) is found at treeline and below treeline elevations. Slabs can fail easily on this layer, either naturally or with the weight of a person or machine. Deeper in the snowpack, an early-season rain crust and sugary facets exist. An avalanche in motion could step down to these deeper layers, creating a large and destructive avalanche. Although the snowpack structure is variable across the region, these persistent slab problems are generally widespread. Snowpack depths are across the region are similarly variable, but typically shallower in the east.

Problems

Storm Slabs

Thursday's storm formed fresh storm slabs over a new weak layer. Sustained strong winds are likely to make slabs increasingly thick and reactive on Friday. A storm slab release may have the force needed to trigger a deeper persistent weak layer.

If triggered, storm slabs may step down to deeper layers and result in large avalanches.Strong southwest winds will likely form thicker, more sensitive slabs on northeast aspects.Whumpfing, shooting cracks and recent avalanches are all strong indicators of unstable snowpack.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely - Very Likely

Expected Size

1 - 2

Persistent Slabs

Large avalanches may be triggered on a number of buried weak layers, even in fairly dense trees and shallow slope angles. Stress on deep weak layers will increase as strong winds continue to load the snowpack on Friday.

Avoid open slopes and convex rolls where buried weak layers may be preserved.Back off if you encounter whumpfing, hollow sounds, or shooting cracks.Stick to simple terrain and be aware of what is above you at all times.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

1.5 - 3

Valid until: Jan 19th, 2018 2:00PM